Super-Resolution Optical Disc with Radial Density Increased by Narrowed Track Pitch Corresponding to Diffraction Limit

Abstract

In–Sb-based super-resolution near-field structure read-only-memory discs with radial density increased by introducing a narrow track pitch corresponding to the diffraction limit of an optical system were developed. Using an optical system with a laser diode with a wavelength of 405 nm and an objective lens with a numerical aperture of 0.85, we confirmed that differential phase detection (DPD) could detect track errors from disc samples recorded random data including a minimum pit length of 75 nm in a 240 nm track period. It has higher capability of track error detection than push–pull detection at a narrowed track pitch. Moreover, bit error rates satisfying the criterion of 3.0×10-4 were experimentally obtained for 66.7-GB-capacity disc samples with a 240 nm track pitch through signal processing with the partial response maximum likelihood of the (1,2,2,1)-type, by applying DPD to tracking servo control. The feasibility of increasing the track density of the Blu-ray DiscTM physical format by 1.33 times was indicated.